> I just discovered that plotting a function that is in TraditionalForm> (via postfix //TraditionalForm) results in an empty plot. Is this> correct, or is this a mistake?

I presume you meant something like the following...

f[x_] := Cos[x] Plot[f[x] // TraditionalForm, {x, 0, 2 Pi}]

=85 which indeed results in no graph appearing.

But why do that?

If you wish, make the entire Plot cell TraditionalForm; that will work OK -- with a reference to separately defined function f (whether that f was defined in an Input cell that had TraditionalForm or StandardForm) or with the function expression directly embedded as the first argument.

Or, if you wish, make just the definition of your function in TraditionalForm. Then use a StandardForm Plot expression.